LightSquared's plan to build a nationwide 4G network was hit by what seemed like a death blow in February, when the Federal Communications Commission revoked its tentative approval because the network would interfere with GPS devices. But LightSquared has influential friends, and lawmakers were eager to blame the FCC, even if they didn't really understand the technical reasons that caused LightSquared's proposal to fail. Spectrum swaps with federal users to bail out the company have also been proposed.

LightSquared came up with its own proposal on Friday (PDF), which involves sharing 5MHz of spectrum with the federal government in a band partially used for weather balloons. It also involves abandoning its most controversial piece of spectrum, the one just below the GPS signals. But LightSquared still wants to build on the lower 10MHz of its spectrum, even though the FCC has already rejected it as unsuitable because millions of existing GPS devices can't filter out neighboring signals. To compensate, LightSquared would lower the power levels in this portion of spectrum to 52dBm per sector EIRP, down from the previously planned power level of 62dBm, which was found to interfere with GPS.

Specifically, LightSquared is proposing to relinquish any claim on the 10MHz (1545-1555 MHz) closest to the GPS band. It is simultaneously asking for rule changes (PDF) that would permit its "eventual robust commercial use of that lower 10MHz of spectrum," a downlink band at 1526-1536 MHz.

This isn't much of a concession. LightSquared already agreed to temporarily give up on deploying in the upper 10MHz. But that wasn't sufficient. "Based on the testing and analyses conducted to date, as well as numerous discussions with LightSquared, it is clear that LightSquared's proposed implementation plans, including operations in the lower 10MHz, would impact both general/personal navigation and certified aviation GPS receivers," the National Telecommunications and Information Administration (NTIA) told the FCC in February. That information led to the withdrawal of LightSquared's conditional approval to deploy terrestrial broadband services.

LightSquared's approval from the previous year had been conditioned on preventing interference with GPS devices. The NTIA did find that LightSquared could prevent interference with GPS by adjusting power levels, antenna height, and antenna angles, but LightSquared rejected this as impractical. Such modifications would prevent the proposed network from hitting appropriate service levels without an investment of billions to build more base stations, which would in any case cause more interference with GPS receivers.

LightSquared "volunteers" to delay use of spectrum it can't use anyway

LightSquared does seem resigned to at least some further delay in deploying on the lower 10MHz, as it says it would "voluntarily" not deploy on that lower 10MHz while the FCC conducts the potential rulemaking proceeding. Since the FCC has already explicitly told LightSquared it cannot deploy on this chunk of spectrum, the use of the word "voluntarily" is an interesting one.

LightSquared hopes its concessions, including the lower power level and some geographic restrictions, will allow limited use of the lower 10MHz until it becomes viable to proceed with "robust terrestrial use of that spectrum." But robust deployment could well require major changes to GPS receiver standards, and replacement of millions of existing GPS devices already deployed for personal and public safety use. Even if a government plan to overhaul GPS standards is devised, it would take many years to implement because of the huge GPS install base.

The new part of LightSquared's proposal involves deploying terrestrial services on 1670-1675 MHz, which LightSquared already has authority to use nationwide, and on 1675-1680 MHz, which LightSquared wants to share with federal government users. This would create a contiguous 10MHz of spectrum for downlink to get the network started, while LightSquared would use its uplink bands of 1627.5-1637.5 MHz and 1646.7-1656.7 MHz.

The 1675-1680 spectrum is "used for federal purposes including National Oceanographic and Atmospheric Administration weather balloons," the IDG News Service reports. The FCC is moving ahead with spectrum sharing—but getting spectrum from the government is not easy, and the federal spectrum LightSquared wants isn't part of the 1,000MHz the government is targeting for sharing.

"I suppose if they avoid using bands that cause problems for GPS and are able to figure out some way of sharing government spectrum, then the proposal is technically feasible," wireless researcher Peter Rysavy told Ars via e-mail. "However, making sharing work is potentially very complicated, so it could be a long-term process to figure out the mechanics. Beyond technology, the politics of getting the spectrum swapped could also be very challenging."

The lower 10MHz did cause problems for GPS in government testing, as we noted earlier, and GPS makers have opposed LightSquared's plans.

In a response to LightSquared's latest proposal, GPS maker Trimble said it "supports the initiation of a rulemaking proceeding to consider the appropriate long term use of the mobile satellite spectrum adjacent to GPS," but wants to avoid paying lots of money to overhaul GPS devices.

"The issue is not merely whether there are ‘technology solutions’ to interference issues, but whether or not those solutions can be implemented at reasonable cost and without substantially hindering future innovation in location-based products and services," Trimble VP and general counsel Jim Kirkland said in a statement e-mailed to Ars. "Moreover, consistent with established FCC policy, any such costs must be borne by the new spectrum use, not US businesses and consumers who rely on GPS." Kirkland did not say whether LightSquared's newly proposed power levels are low enough to prevent interference.

LightSquared's friends in Congress want the FCC to get its network back on track immediately. US Rep. Cliff Stearns (R-FL) recently demanded a solution from FCC officials, but FCC Chief of Engineering and Technology Julius Knapp said LightSquared's fate is subject to an open proceeding, and it's too early to endorse any one proposal.

Congressional Republicans have been split on LightSquared, a story in The Hill notes, with some blasting President Obama for helping LightSquared too much, and others accusing Obama's administration of deliberately torpedoing LightSquared's proposal.